Is there gravity in deep space?

Newton's law of universal gravitation says that every mass attracts every other mass with a force that falls off as the square of the distance between them. Squaring a large number makes that force very small, but it never makes it zero. A spacecraft halfway between the stars still feels the Sun behind it and the mass of the Milky Way around it. That is why the Voyager probes, far beyond the orbit of Pluto, still follow paths curved by the Sun rather than travelling in perfectly straight lines.

The weightlessness seen aboard the International Space Station is often blamed on an absence of gravity, which is wrong. At the station's altitude of roughly 400 km, Earth's pull is still about 90% of its strength at sea level, where it produces an acceleration of about 9.8 m/s2. The crew floats because the station is in continuous free fall around the planet: vehicle, people and water droplets all accelerate together, so nothing presses against anything else. Engineers call what remains microgravity - the tiny residual accelerations from atmospheric drag, thruster firings and crew movement.

Deep space does differ from low Earth orbit in one respect: no single body dominates. Near Earth the planet sets the local acceleration; between the planets the Sun does; out in interstellar space the combined pull of the Galaxy takes over and local accelerations become minute. Those faint forces still govern navigation, because over months and years they bend trajectories enough to decide whether a probe arrives at its target or misses it entirely.